The B-B bond activation of the diborane(4) derivatives B(2)cat(2) with the copper(I) alkoxido complex [(SIDipp)Cu-OtBu] delivers, depending on the solvent, either the linear boryl complex [(SIDipp)Cu-Bcat] from PhMe or the mu-boryl complex [((SIDipp)Cu)(2)Bcat][cat(2)B] from THF. The relevant conversion of the linear boryl complex to the mu-boryl complex occurs in the polar solvent via formal boryl anion abstraction by the Lewis acid catB-OtBu, concomitantly formed during the B-B activation. With Lewis acids such as BPh3 or [CPh3[BArF] (reversible), boryl abstraction from the linear complexes [(SIDipp)Cu-Bcat] or [(SIDipp)Cu-Bdmab] occurs and results in the mu-boryl complexes [((SIDipp)Cu)(2)Bcat/dmab] [Ph3B-Bcat/dmab] and [((SIDipp)Cu)(2)Bcat][BArF]. The formation of [((SIDipp)Cu)(2)Bcat][cat(2)B] is generally accompanied by the concomitant formation of the mu-hydrido complex [((SIDipp)Cu)(2)H][cat(2)B]. The spiroborate [cat(2)B](-) is formed from the initially formed Lewis acid/base adduct [catB-B(OtBu)cat](-) presumably in a process that involves the glass surface of the reaction vessel. All complexes are thoroughly characterized structurally as well as spectroscopically, in particular with respect to the dynamic behavior of the mu-boryl complexes in solution.